size(500, 500);

int numLevels = 10;
float stepSize = height/(1+numLevels);

for (int level = 0; level < numLevels; level++) {
  fill(level * 255 / numLevels, 0, 0);
  rect(0.5 * stepSize, stepSize * (0.5 + level), stepSize * (1+level), stepSize);
}